A particle counter accumulates the number of particles detected during a period of time. An application of a particle counter may be found in equipment of a manufacturing process which is sensitive to free particles, such as semiconductor manufacturing equipment.
Typically, a timer in the particle counter, or its controller, controls the beginning and end points of a count period. At the end of the count period, the particle count is displayed and stored. Thereafter, the particle counter is reset prior to particle counting in the next period. Such particle counter expresses the count in units of, for example, "counts per minute." Thus, the count period is a basic parameter of standard particle counting. However, when a particle counter is attached to a piece of process equipment which can be used in numerous process variations, simple time-based particle counting has a number of disadvantages.
For example, when the particle counter is used to control a process (e.g. in a "statistical process control" application), the number of particles per process event must be captured and analyzed. If the particle counter is not initially synchronized to process events, the resulting particle counts re accumulated over the duration of fixed time periods bearing little relation to the process events. Also, a process event may be of variable duration, so that the number of fixed intervals will vary. In that situation, a "particles-per-event" count is necessary for process control purposes. Thus, particle counts obtained using the prior art methods cannot be used at a later time to relate the particles detected to the process events within the process cycle.
To obtain finer resolution of particle counts when a number of process events are occurring over the count period, time-based particle counting over short time intervals generates too much data. Conversely, long count periods in these applications result in poor resolution. Further, because count periods are out of step with process events, it may be difficult to use the count data for an application such as isolating the process event which contributes most to the particle count.
In certain applications, it is desirable not only to determine which process event causes particle generation, but also whether the particles are generated at the beginning, at the intermediate, or at the end phases of a process event. Such information can often facilitate troubleshooting the process equipment for detecting the source of particle generation.
Another disadvantage of time-based particle counting results from running the particle counter regardless of whether significant process events are taking place inside the process equipment, thereby causing much useless data to be acquired and stored even during equipment dead time.